The present invention relates to an electrochromic display device used in a variety of display applications for timepieces, measuring instruments, household electric appliance, etc.
In contrast to the liquid crystal display, the electrochromic display features a wide-viewing angle, bright color and a high degree of definition. The well known inorganic electrochromic material is tungsten oxide or WO.sub.3. The most well known organic electrochromic material is viologen, and other coloring materials such as pyrazolines, anthraquinones and the like are also known.
The device is made by forming a thin film of WO.sub.3 on each of a pair of transparent electrodes by evaporation or the like and providing an electrolyte or a dielectric film between the opposed electrodes. From a practical point of view, the use of WO.sub.3 is disadvantageous in that the display life is short, there are variations in color among the different display segments and only a single kind of coloration color or bluish color is possible. There are other disadvantages in that the material of the opposed electrodes can react and a reflecting plate or the like must be incorporated in the device.
On the other hand, while the organic coloring materials have the possibility of ensuring various kinds of coloration colors as compared with the inorganic materials, generally they also suffer the problem of display life. The organic coloring material of the viologen type produces color by reduction and returns to the bleached state upon its oxidation. The coloring material of the viologen type has the disadvantage of a short display life. The reason is that since the colored coloring material becomes insoluble to the solvent, the coloring material exhibits both solubility and insolubility phenomena in accordance with its bleaching and coloration and this reversability is responsible for its short life. Also, since the ions take part in these oxidation and reduction reactions, these ions produce a detrimental effect on the transparent electrodes in some cases and there is also the disadvantage of a large power consumption. It will thus be seen that the existing electrochromic display has disadvantages in terms of the display life and response speed characteristics over the liquid crystal display.
Generally, the electrochromic display device is constructed so that the display electrode is formed on one of a pair of glass substrates and a counter electrode is formed on the other glass substrate with an electronic material being sealed between the electrodes. It is only necessary that at least one of the glass substrates be made of a transparent material. Suitable electrode materials for the display electrode and the counter electrode include such transparent electrodes as In.sub.2 O.sub.3 and SnO.sub.2. The area of the display electrode is smaller than that of the counter electrode. Epoxy resin, low melting point glass, solder or the like is used as a sealing material and filling material. A display producing substance is a solution prepared by dissolving a coloring material such as styryl type analogous compound and a supporting electrolyte in a non-aqueous organic solvent. With this construction, while the display life of the device is determined by some factors such as the impurities included in the display producing substance, one of the factors is a change of the properties of the counter electrode material by an electrochemical reaction. In other words, since each of the display electrode and the counter electrode is made of a transparent electrode of a metal oxide such as In.sub.2 O.sub.3, it tends to be subjected to a reduction reaction so that the reduction reaction causes the elimination of oxygen atoms and hence oxidation of the ingredients in the solution thereby causing a detrimental effect on the display life. The electrodes tend to undergo the reaction more with an increase in the current value. In particular, the change of the properties of the counter electrode is a problem.
The change of properties of the counter electrode is particularly severe in the portion facing the display electrode so that the reduction occurs materially and the display life is deteriorated greatly.
To overcome this drawback, it has recently been proposed to form an insulating film on the portion of the counter electrode facing the display electrode. While this has the effect of decreasing the amount of current flow to the counter electrode and thus improving the display life to some extent, the effect is still insufficient.
Further, where a dc voltage is applied across the display electrode and the counter electrode of the electrochromic display device in a manner that the display electrode becomes positive, there is a disadvantage that a longer time is required before the coloration density reaches a saturation state and thus the response speed is low.